The invention relates to a screw capper which screws and tightens a cap on a vessel, and in particular, to a screw capper which screws and tightens a cap on a vessel formed of plastics.
In a screw capper directed to a vessel formed of plastics such as PET bottle or the like, a difficulty in constraining a readily deformable vessel with a gripper has been recognized, and in order to accommodate for this, a cap is screwed and tightened while applying a load on the vessel through the cap. An engaging pawl is caused to bite into the lower surface of a flange provided on the vessel in order to prevent the vessel from rotating during the screwing and tightening operation.
Specifically, the cap is strongly urged against the vessel when a thread formed on the cap is urged against a thread formed on the vessel. If the relative positions of the beginning turns of the threads on the cap and the vessel were as shown in FIG. 4 such that a thread 4N on the cap and a thread 2N on the vessel are overlaid upon each other only at their distal ends, the both threads have reduced pressure responsive areas where the load is concentrated, giving rise to the likelihood that the cap thread 4N may be broken and its fragments may be dispersed to be contained inside the vessel. If the cap thread were not broken, it may climb up the thread on the vessel to be obliquely mounted thereon in the manner of cocked cap.
To overcome this problem, Japanese Laid-Open Patent Application No. 72,984/1996 discloses a proposal to reduce the load applied to the cap by the provision of the engaging pawl mentioned above in combination with an anti-rotation member, formed of rubber or soft resin, which abuts against the barrel of the vessel during the screwing and tightening operation. However, rubber and/or soft resin are readily abradable and the abrasion of these members has the risk of allowing the vessel to rotate, requiring a frequent replacement of the anti-rotation member. Depending on the configuration of the vessel, the latter may be scarred or crashed.
In view of the foregoing, there is provided a screw capper including conveying means which conveys a vessel, and a capping head which screws and tightens a cap onto the mouth of a vessel which is being conveyed by the conveying means, the capping head comprising cap holder means, rotating means for rotating the cap holder means, elevating means for elevating the holder means, and load imparting means for imparting a load on the holder means which is directed toward a vessel which is located below the holder means.
In accordance with the present invention, the screw capper further comprises means for detecting an angle through which the holder means has rotated, and a controller for controlling the load imparted by the load imparting means. The controller controls the load imparted by the load imparting means to a low value from the beginning of the screwing and tightening operation until the angle through which the holder means has rotated exceeds a specific angle where the load is changed, and controls the load imparted by the load imparting means to a high value in the course of the screwing and tightening operation upon detecting that the angle of rotation of the holder means detected by the angle detecting means has exceeded the specific angle.
With the described arrangement, before the specific angle is exceeded, the load imparted to the holder means is maintained low to enable the screwing and tightening operation to be carried out while avoiding any damage to the threads and preventing the occurrence of cocked cap. After the specific angle is exceeded, the load imparted to the holder means is increased to prevent the vessel from rotating as the screwing and tightening operation is continued. In this manner, a screwing and tightening operation can be achieved in a reliable manner while preventing the vessel from rotating and while avoiding any damage to the threads and the occurrence of cocked cap which would occur otherwise.
Above and other objects, features and advantages of the invention will become apparent from the following description of an embodiment thereof with reference to the attached drawings.